1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device that is electrically writable, readable, and erasable, and its manufacturing method. In particular, the present invention relates to a structure of a floating gate in the nonvolatile semiconductor memory device.
2. Description of the Related Art
The market is expanding for nonvolatile memories in which data can be electrically rewritten and data can be stored even after the power is turned off. Features of a nonvolatile memory lie in that its structure is similar to that of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a region capable of accumulating charges for a long period of time is provided over a channel forming region. Such a charge accumulating region is also referred to as a floating gate since it is formed over an insulating layer and is insulated and isolated from its surroundings. Since the floating gate is surrounded by an insulator and is electrically insulated from the surroundings, there is a feature that the floating gate retains charges that are injected thereto. A gate electrode called a control gate is further provided over the floating gate with an insulating layer interposed therebetween. The control gate is distinguished from the floating gate by application of predetermined voltage to the control gate in data writing or reading.
In a so-called floating gate type nonvolatile memory having such a structure, data is stored through electrical control of charge injection to and discharge from the floating gate. Specifically, charges are injected to and discharged from the floating gate by application of high voltage between a semiconductor layer in which a channel forming region is formed and a control gate. It is said that Fowler-Nordheim (F-N) type tunnel current (NAND type) or hot electrons (NOR type) flow at this time through an insulating layer over the channel forming region at this time. Accordingly, the insulating layer is also referred to as a tunnel insulating layer.
The floating gate type nonvolatile memory is required to have a property, for securing reliability, that charges accumulated in the floating gate can be retained for 10 years or more. Accordingly, the tunnel insulating layer is required to have a thickness that allows a tunnel current to flow and to have a high insulating property so that charges do not leak.
In addition, the floating gate formed over the tunnel insulating layer is formed using silicon that is the same semiconductor material as that of the semiconductor layer in which the channel forming region is formed. Specifically, a method for forming the floating gate using polycrystalline silicon has been widely used, and for example, a polysilicon film deposited with a thickness of 400 nm is known (refer to Patent Document 1: Japanese Published Patent Application No. 2000-58685, p 7, FIG. 7)